Power conversion device and power conversion method

ABSTRACT

A power conversion device includes three inverters configured to convert DC power of DC buses into single-phase AC power, and a controller configured to control the three inverters so as to generate three-phase AC power. The controller is configured to generate a fundamental wave command for generating one-phase AC power constituting the three-phase AC power, and to generate an adjustment wave command having triple the frequency of the fundamental wave command. Additionally, the controller is configured to output a phase voltage command, in which the adjustment wave command is superimposed on the fundamental wave command, and to determine an initial phase of the adjustment wave command to be offset from an initial phase of the fundamental wave command so as to reduce a voltage ripple occurring in the DC buses at double the frequency of the fundamental wave command.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2018-077533, filed on Apr. 13, 2018, theentire contents of which are incorporated herein by reference.

BACKGROUND 1. Field

The present disclosure relates to a power conversion device and a powerconversion method.

2. Description of the Related Art

Japanese Unexamined Patent Publication No. H7-135797 discloses aso-called multiple inverter which increases the output capacity bycombining the outputs of a plurality of inverters.

SUMMARY

A power conversion device according to one aspect of the presentdisclosure includes: three inverters each of which is configured toconvert DC power of a DC bus into single-phase AC power; and acontroller configured to control the three inverters so as to generatethree-phase AC power. The controller may include a fundamental wavegeneration module configured to generate a fundamental wave command forgenerating one-phase AC power constituting the three-phase AC power foreach of the inverters, and an adjustment wave generation moduleconfigured to generate an adjustment wave command having triple thefrequency of the fundamental wave command for each of the inverters.Additionally, the controller may include a command output moduleconfigured to output a phase voltage command, in which the adjustmentwave command is superimposed on the fundamental wave command, for eachof the inverters, and a phase calculation module configured to calculatea phase of the adjustment wave command based on a power factor of thethree-phase AC power so as to reduce a voltage ripple occurring in theDC bus at double the frequency of the fundamental wave command.

A power conversion method according to another aspect of the presentdisclosure includes generating a fundamental wave command for generatingone-phase AC power of three-phase AC power for each of three inverters,such that the three-phase AC power is generated in the three invertersconfigured to convert DC power of a DC bus into single-phase AC power,and generating an adjustment wave command having triple the frequency ofthe fundamental wave command for each of the inverters. Additionally,the method may include outputting a phase voltage command, in which theadjustment wave command is superimposed on the fundamental wave command,for each of the inverters, and calculating a phase of the adjustmentwave command with respect to the fundamental wave command based on apower factor of the three-phase AC power so as to reduce a voltageripple occurring in the DC bus at double the frequency of thefundamental wave command.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an overall configuration of apower conversion device;

FIG. 2 is a schematic diagram illustrating a configuration of aninverter cell and a controller;

FIG. 3 is a graph showing a relationship between a fundamental waveamplitude and an adjustment wave amplitude;

FIG. 4 is a block diagram illustrating a hardware configuration of acontroller;

FIG. 5 is a block diagram illustrating a power conversion procedure;

FIG. 6 is a block diagram illustrating a procedure of calculating anamplitude of an adjustment wave;

FIG. 7 is a block diagram illustrating a procedure of calculating aphase of an adjustment wave;

FIG. 8 is a block diagram illustrating an overmodulation correctionprocedure;

FIG. 9 is a block diagram illustrating a procedure of generating a boostcommand;

FIG. 10 is a flow chart illustrating an example power conversion methodfor generating AC power; and

FIG. 11 is a flow chart illustrating another example power conversionmethod for generating AC power.

DETAILED DESCRIPTION

Hereinafter, embodiments will be described in detail with reference tothe drawings. The same elements or the elements having the same functionare denoted by the same reference numerals and a redundant descriptionthereof will be omitted.

(Power Conversion Device)

A power conversion device 1 illustrated in FIG. 1 is a system forgenerating three-phase AC power to be supplied to a load of an electricmotor or the like. The power conversion device 1 includes threeinverters 2 and a controller 100. Each of the three inverters 2 convertsDC power of a DC bus into single-phase AC power and supplies thesingle-phase AC power to a load (for example, an electric motor 9) asone phase of three-phase AC power. Hereinafter, the inverters 2 forU-phase, V-phase, and W-phase are separately referred to as an inverter2U, an inverter 2V, and an inverter 2W, respectively. For example, theU-phase inverter 2U supplies the single-phase AC power to the electricmotor 9 as the U-phase AC power, the V-phase inverter 2V supplies thesingle-phase AC power to the electric motor 9 as the V-phase AC power,and the W-phase inverter 2W supplies the single-phase AC power to theelectric motor 9 as the W-phase AC power. One AC output terminal of eachof the inverters 2U, 2V, and 2W is connected to the electric motor 9,and the other AC output terminal of each of the inverters 2U, 2V, and 2Wis connected to each other. The connection point between the inverters2U, 2V, and 2W corresponds to a neutral point 4 of the three-phase ACpower. A three-phase AC current flowing into the electric motor 9 isdetected by a current detector 5, and a detection signal is input to thecontroller 100. In some examples, the current detectors 5 may beprovided to all three-phases. In other examples, the current detector 5may be provided in two of the three phases, and the current value of theremaining one phase may be calculated by the controller 100.

The electric motor 9 may comprise synchronous electric motors (includingsurface permanent magnet motors (SPMM), interior permanent magnet motors(IPMM)), induction motors, other types of AC electric motors, or anycombination thereof. In addition, the electric motor 9 may comprise arotary electric machine type motor, a linear motor, a power generator,or any combination thereof.

Each of the inverters 2U, 2V, and 2W may be a series connectedmulti-level inverter in which the AC sides of a plurality of invertercircuits are connected in series. For example, each of the inverters 2U,2V, and 2W may include a plurality of inverter cells 10 connected inseries between the neutral point 4 and the electric motor 9. If theplurality of inverter cells 10 are connected in series to each other,the AC sides of inverter circuits 12 (described later) respectivelyincluded in the inverter cells 10 are connected in series to each other.

The controller 100 controls the inverters 2U, 2V, and 2W so as togenerate the three-phase AC power. For example, the controller 100controls the plurality of inverter cells 10 of the inverter 2U so as togenerate the single-phase AC power for the U phase, controls theplurality of inverter cells 10 of the inverter 2V so as to generate thesingle-phase AC power for the V phase, and controls the plurality ofinverter cells 10 of the inverter 2W so as to generate the single-phaseAC power for the W phase. The controller 100 and each inverter cell 10are connected by communication links 6. In a case where the inverters2U, 2V, and 2W are series connected multi-level inverters, opticalcommunication devices or the like may be used as the communication links6 so as to electrically insulate the controller 100 and the invertercells 10 from each other. Hereinafter, example configurations of theinverter cells 10 and the controller 100 will be described in furtherdetail.

(Inverter Cell)

The inverter cell 10 converts AC power supplied from an AC power sourceinto single-phase AC power for driving the electric motor 9. Asillustrated in FIG. 2, the inverter cell 10 includes a rectifier circuit11, an inverter circuit 12, a capacitor 13, a voltage detector 31, and acell controller 40.

The rectifier circuit 11 is, for example, a diode bridge circuit, andconverts AC power from the AC power source into DC power and outputs theDC power to the DC buses 14P and 14N. The capacitor 13 is connectedbetween the DC buses 14P and 14N and smoothes the DC voltage between theDC buses 14P and 14N.

The inverter circuit 12 is a circuit which converts DC power of the DCbuses 14P and 14N into single-phase AC power. The inverter circuit 12includes a switching circuit 16 and a gate driving circuit 15. Theswitching circuit 16 includes a plurality of switching elements 17 (forexample, four switching elements) and converts DC power into AC power byswitching on/off the switching elements 17. The switching element 17 is,for example, a power metal oxide semiconductor field effect transistor(MOSFET), an insulated gate bipolar transistor (IGBT), or the like, andswitches on/off in accordance with a gate driving signal. Whileinsulating the cell controller 40 from the switching circuit 16, thegate driving circuit 15 converts the gate driving signal input from thecell controller 40 into a signal form capable of driving the switchingelement 17 of the switching circuit 16 and outputs the signal to theswitching element 17.

The voltage detector 31 detects a DC voltage between the DC buses 14Pand 14N.

The cell controller 40 provides input and output via signalcommunications with the controller 100 and performs control processingof each part of the inverter cell 10. For example, the cell controller40 generates the gate driving signal in response to a command (forexample, a phase voltage command) from the controller 100, and outputsthe gate driving signal to the gate driving circuit 15. In addition, thecell controller 40 acquires the detection result of the voltage detector31 and outputs the detection result to the controller 100 as necessary.

(Controller)

The controller 100 is configured to generate a fundamental wave commandfor generating one-phase AC power of three-phase AC power for each ofthe inverters 2U, 2V, and 2W so as to cause the inverters 2U, 2V, and 2Wto generate three-phase AC power. Additionally, the controller 100 isconfigured to generate an adjustment wave command having triple thefrequency of the fundamental wave command for each of the inverters 2U,2V and 2W, and output a phase voltage command, in which the adjustmentwave command is superimposed on the fundamental wave command, for eachof the inverters 2U, 2V, and 2W. Still further, the controller 100 isconfigured to calculate the phase of the adjustment wave command basedon the power factor of the three-phase AC power so as to reduce thevoltage ripple occurring in the DC buses 14P and 14N at double thefrequency of the fundamental wave command.

For example, the controller 100 may include, as a functionalconfiguration (hereinafter referred to as a “function module”), acurrent command generation module 111, a fundamental wave generationmodule 112, an adjustment wave generation module 113, a command outputmodule 114, a coordinate conversion module 115, a power factor anglecalculation module 121, a phase calculation module 122, an amplitudecalculation module 123, and a magnetic pole position informationacquisition module 131.

The current command generation module 111 generates a current command(current target value) in accordance with a torque target value. Forexample, when the power conversion device 1 performs the speed controlof the electric motor 9, the torque target value is determined inaccordance with a deviation between the speed target value and the speedpresent value. When the power conversion device 1 performs the torquecontrol of the electric motor 9, the control target value becomes theabove-mentioned torque target value. For example, the current commandgeneration module 111 generates a current command in a two-dimensionalcoordinate system fixed to a rotor of the electric motor 9. Morespecifically, the current command generation module 111 generatescurrent command values Id_ref and Iq_ref in a dq coordinate system. Inthe dq coordinate system, for example, in a case where the electricmotor 9 is a synchronous electric motor, a magnetic pole direction is ad axis and a direction orthogonal thereto is a q axis.

The fundamental wave generation module 112 generates a fundamental wavecommand for generating one-phase AC power constituting the three-phaseAC power for each of the inverters 2U, 2V, and 2W. For example, thefundamental wave generation module 112 generates the fundamental wavecommand for the inverter cells 10 for each of the inverters 2U, 2V, and2W. In some examples, the fundamental wave generation module 112 equallydivides the fundamental wave command for generating the U-phase AC powerin accordance with the current command values Id_ref and Iq_ref by thenumber of inverter cells 10 belonging to the inverter 2U, and generatesthe fundamental wave command for the inverter cell 10 of the inverter2U. Similarly, the fundamental wave generation module 112 may equallydivide the fundamental wave command for generating the V-phase AC powerin accordance with the current command values Id_ref and Iq_ref by thenumber of inverter cells 10 belonging to the inverter 2V, and generatethe fundamental wave command for the inverter cell 10 of the inverter2V. Additionally, the fundamental wave generation module 112 may equallydivide the fundamental wave command for generating the W-phase AC powerin accordance with the current command values Id_ref and Iq_ref by thenumber of inverter cells 10 belonging to the inverter 2W, and generatethe fundamental wave command for the inverter cell 10 of the inverter2W.

In some examples where the electric motor 9 is a synchronous motor, thefundamental wave generation module 112 generates a fundamental wavecommand Kbase per the following expressions for each of the inverters2U, 2V, and 2W.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 1} \right\rbrack & \; \\\left\{ \begin{matrix}{{Kbase\_ u} = {K \cdot {\sin\left( {\omega_{0} \cdot t} \right)}}} \\{{Kbase\_ v} = {K \cdot {\sin\left( {{\omega_{0} \cdot t} - {\frac{2}{3} \cdot \pi}} \right)}}} \\{{Kbase\_ w} = {K \cdot {\sin\left( {{\omega_{0} \cdot t} - {\frac{4}{3} \cdot \pi}} \right)}}}\end{matrix} \right. & (1)\end{matrix}$

ω₀: angular frequency of fundamental wave command

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack & \; \\{K = \frac{A}{Vdc}} & (2)\end{matrix}$

A: voltage command amplitude in accordance with current command valuesId_ref and Iq_ref, current measurement values Id_fbk and Iq_fbk(described later), frequency command

Vdc: detected value of DC voltage by voltage detector 31

Although Expressions (1) and (2) exemplify the case where the voltageamplitude of the fundamental wave is calculated as a ratio to the DCvoltage value Vdc between the DC buses 14P and 14N, the fundamental wavegeneration module 112 may calculate the voltage amplitude of thefundamental wave command as the absolute value of the voltage.

The adjustment wave generation module 113 generates the adjustment wavecommand having triple the frequency of the fundamental wave command. Thephase of the adjustment wave command is calculated for each of theinverters 2U, 2V, and 2W with reference to the phase of the fundamentalwave command. 120°, which is the phase difference between the phases ofthe fundamental wave command, corresponds to 360° in the adjustment wavecommand. Therefore, since the phase difference between the phases of theadjustment wave command is 360°, the phase of the adjustment wavecommand becomes equal for the inverters 2U, 2V, and 2W. For example, theadjustment wave generation module 113 may be configured to equallygenerate an adjustment wave command Kadd per the following expressionfor all the inverter cells 10.[Expression 3]Kadd=K_3·sin(ϕ_3)  (3)

K_3: amplitude of adjustment wave command (hereinafter referred to as“adjustment wave amplitude”)

φ_3: phase of adjustment wave command (hereinafter referred to as“adjustment wave phase”)

The adjustment wave amplitude K_3 is calculated by the amplitudecalculation module 123 described later, and φ_3 is calculated by thephase calculation module 122 described later.

The command output module 114 outputs the phase voltage command, inwhich the adjustment wave command is superimposed on the fundamentalwave command, for each of the inverter cells 10. Outputting the phasevoltage command also includes outputting a modulation factor valueindicating the magnitude of the phase voltage target value by the ratioto the DC voltage value Vdc between the DC buses 14P and 14N. Forexample, the command output module 114 outputs the modulation ratevalue, in which the adjustment wave command Kadd calculated byExpression (3) is added to the fundamental wave command Kbase calculatedby Expression (1), to the cell controller 40 of each inverter cell 10 asthe phase voltage command. The cell controller 40 generates the gatedriving signal in accordance with the phase voltage command and outputsthe gate driving signal to the gate driving circuit 15.

In some examples, the amplitude of the phase voltage command may becompared with the DC voltage value Vdc. In a case where the phasevoltage command is indicated by the ratio to the DC voltage value Vdc,the DC voltage value Vdc to be compared with the amplitude of the phasevoltage command is 1.0. For example, if the amplitude of the phasevoltage command exceeds the DC voltage value Vdc, the amplitude of thephase voltage command exceeds 1. In addition, if the amplitude of thephase voltage command is less than the DC voltage value Vdc, theamplitude of the phase voltage command is less than 1.

The coordinate conversion module 115 acquires the U-phase currentmeasurement value Iu_fbk, the V-phase current measurement value Iv_fbk,and the W-phase current measurement value Iw_fbk from the currentdetectors 5, respectively, and converts the current measurement valuesIu_fbk, Iv_fbk, and Iw_fbk into current measurement values Id_fbk andIq_fbk in the dq coordinate system.

The power factor angle calculation module 121 calculates the powerfactor angle of the three-phase AC power output from the inverters 2U,2V, and 2W. The power factor angle φ is calculated by subtracting thecurrent phase angle formed by the current measurement values Id_fbk andIq_fbk from the voltage phase angle formed by the voltage values Vd andVq, which represent the output voltages of the inverters 2U, 2V, and 2Win the dq coordinate system. For example, the power factor anglecalculation module 121 calculates the power factor angle φ by thefollowing expression.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 4} \right\rbrack & \; \\{\varphi = {{\tan^{- 1}\left( \frac{Vq}{Vd} \right)} - {\tan^{- 1}\left( \frac{Iq\_ fbk}{Id\_ fbk} \right)}}} & (4)\end{matrix}$

The power factor angle calculation module 121 may use the voltage valuesVd and Vq based on the actual measurement result of the output voltageto calculate the power factor angle, or may use the voltage commandvalues Vd_ref and Vq_ref as the voltage values Vd and Vq.

The phase calculation module 122 calculates the phase of the adjustmentwave command based on the power factor of the three-phase AC power so asto reduce the voltage ripple occurring in the DC buses 14P and 14N atdouble the frequency of the fundamental wave command (hereinafterreferred to as “second harmonic ripple”). In some examples, the phasecalculation module 122 may determine an initial phase of the adjustmentwave command to be offset from (in other words, to be different from) aninitial phase of the fundamental wave command. The initial phase of theadjustment wave command is a phase of the adjustment wave command at atime when the phase of the fundamental wave command is equal to zero.For example, the phase calculation module 122 calculates the phase φ_3based on the following expression.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 5} \right\rbrack & \; \\\left\{ \begin{matrix}{{{\phi\_}3} = {{3 \cdot \omega_{0} \cdot t} - \theta_{N}}} \\{\theta_{N} = {2 \cdot \varphi}}\end{matrix} \right. & (5)\end{matrix}$

The initial phase θ_(N) may be different from double the power factorangle as long as it is calculated so as to reduce at least the secondharmonic ripple. For example, in a case where a value corresponding todouble the power factor angle is set as a reference value θ_(N_ref), theabsolute value of the difference between the reference value θ_(N_ref)and the initial phase θ_(N) has only to be less than at least theabsolute value of the reference value θ_(N_ref). The absolute value ofthe difference between the reference value θ_(N_ref) and the initialphase θ_(N) may be 10% or less, 5% or less, or 3% or less of theabsolute value of the reference value θ_(N_ref).

The amplitude calculation module 123 calculates the amplitude of theadjustment wave command based on the amplitude of the fundamental wavecommand so as to reduce the above-mentioned second harmonic ripple. Forexample, the amplitude calculation module 123 calculates the adjustmentwave amplitude K_3 to have the same value as the fundamental waveamplitude K. The adjustment wave amplitude K_3 may be different from thefundamental wave amplitude K as long as it is calculated so as to reduceat least the second harmonic ripple. For example, the absolute value ofthe difference between the fundamental wave amplitude K and theadjustment wave amplitude K_3 has only to be less than at least thefundamental wave amplitude K. The absolute value of the differencebetween the fundamental wave amplitude K and the adjustment waveamplitude K_3 may be 50% or less, 30% or less, or 10% or less of thefundamental wave amplitude K.

Here, as the fundamental wave amplitude K of the fundamental wavecommand Kbase increases, the amplitude of the phase voltage command mayexceed the DC voltage value Vdc by superimposing the adjustment wavecommand Kadd on the fundamental wave command Kbase. The inverter circuitmay, in principle, be unable to generate the AC voltage having theamplitude exceeding the DC voltage value Vdc between the DC buses 14Pand 14N. In that case, if the phase voltage command having the amplitudeexceeding the DC voltage value Vdc (that is, the phase voltage commandexceeding 1) is output to the inverter circuit, such that the desiredthree-phase AC power is not obtained, it may be difficult to control theelectric motor 9 to a desired state.

FIG. 3 is a graph showing the calculation example of the adjustment waveamplitude K_3 in consideration of the above. In the graph, thehorizontal axis represents the value of the fundamental wave amplitude Kand the vertical axis represents the value of the adjustment waveamplitude K_3 calculated in accordance with the fundamental waveamplitude K. A line L1 shows the calculation example when the powerfactor is 1 (when the power factor angle is zero). In the ascending lineL11 of the line L1, the adjustment wave amplitude K_3 is calculated tobe the same value as the fundamental wave amplitude K. A modulation ratethreshold value Kth represents the value of the fundamental waveamplitude K at which the phase voltage command becomes 1 when theadjustment wave amplitude K_3 is calculated in accordance with theascending line L11. In a situation in which the fundamental waveamplitude K exceeds the modulation rate threshold value Kth, if theadjustment wave amplitude K_3 is set to the same value as thefundamental wave amplitude K, the phase voltage command will exceed 1.Therefore, in a descending line L12 of the line L1, the value of theadjustment wave amplitude K3 is adjusted such that the phase voltagecommand becomes 1.

In this manner, in order to switch the calculation method of theadjustment wave amplitude K_3 in accordance with the value of thefundamental wave amplitude K, the amplitude calculation module 123 mayinclude a first amplitude calculation module 125, a second amplitudecalculation module 126, and an amplitude selection module 127 asillustrated in FIG. 2. The first amplitude calculation module 125 may beconfigured to modify the amplitude of the adjustment wave command byincreasing the amplitude of the adjustment wave command in a firstamplitude modification operation in response to an increase in theamplitude of the fundamental wave command in a first range.Additionally, the second amplitude calculation module 126 may beconfigured to reduce the amplitude of the adjustment wave command in asecond amplitude modification operation in response to an increase inthe amplitude of the fundamental wave command in a second range. In someexamples, the controller switches between the first amplitudemodification operation and the second amplitude modification operationin response to the amplitude of the fundamental wave command exceeding apredetermined threshold.

The first amplitude calculation module 125 calculates the adjustmentwave amplitude K_3 based on the fundamental wave amplitude K so as toreduce the second harmonic ripple. For example, the first amplitudecalculation module 125 sets the value of the adjustment wave amplitudeK_3 to the same value as the fundamental wave amplitude K by thefollowing expression.[Expression 6]K_3=K  (6)

The second amplitude calculation module 126 lowers the amplitude of theadjustment wave amplitude K_3 as the fundamental wave amplitude K rises.For example, the second amplitude calculation module 126 calculates theadjustment wave amplitude K_3 by the following expression in accordancewith the straight approximate descending line L21 (see FIG. 3)approximating the descending line L12.[Expression 7]K_3=a·K+b  (7)

a: slope (negative value) of approximate descending line L21

b: intercept of approximate descending line L21

As the fundamental wave amplitude K rises, the amplitude selectionmodule 127 switches the amplitude of the adjustment wave amplitude K_3from the amplitude calculated by the first amplitude calculation module125 (hereinafter referred to as “the first amplitude”) to the amplitudecalculated by the second amplitude calculation module 126 (hereinafterreferred to as “the second amplitude”). For example, as the fundamentalwave amplitude K exceeds a predetermined reference value (hereinafterreferred to as “the first reference value”), the amplitude selectionmodule 127 switches the amplitude of the adjustment wave amplitude K_3from the first amplitude to the second amplitude. The first referencevalue may be, for example, the modulation rate threshold value Kth or avalue less than the modulation rate threshold value Kth.

The value of the modulation rate threshold value Kth changes inaccordance with the power factor of the three-phase AC power.Specifically, the modulation rate threshold value Kth also decreases asthe power factor decreases (as the power factor angle increases).Therefore, the amplitude selection module 127 may change the firstreference value in accordance with the power factor. More specifically,the amplitude selection module 127 may decrease the first referencevalue as the power factor decreases.

The second harmonic ripple is reduced by superimposing the adjustmentwave command, which is generated based on the phase calculated by thephase calculation module 122 and the amplitude calculated by theamplitude calculation module 123, on the fundamental wave command. Thisprinciple will be described below. For simplicity, only the mathematicalexpression in the U phase is shown below.

The output voltage Vbase, the output current Ibase, and the output powerWbase, in accordance with the fundamental wave command, are representedby the following expressions.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 8} \right\rbrack & \; \\{{Vbase} = {A \cdot {\sin\left( {\omega_{0} \cdot t} \right)}}} & (8) \\\left\lbrack {{Expression}\mspace{14mu} 9} \right\rbrack & \; \\{{Ibase} = {B \cdot {\sin\left( {{\omega_{0} \cdot t} - \varphi} \right)}}} & (9) \\\left\lbrack {{Expression}\mspace{14mu} 10} \right\rbrack & \; \\\begin{matrix}{{Wbase} = {{Vbase} \cdot {Ibase}}} \\{= {{- \frac{A \cdot B}{2}} \cdot \left( {{\cos\left( {{2 \cdot \omega_{0} \cdot t} - \varphi} \right)} - {\cos(\varphi)}} \right)}}\end{matrix} & (10)\end{matrix}$

As shown in Expression (10), the frequency of the vibration component ofthe output power Wbase is double the frequency of the fundamental wavecommand Kbase. Therefore, when only the fundamental wave command isoutput as the phase voltage command, the second harmonic ripple occursin the DC voltage value Vdc in accordance with the frequency of theoutput power Wbase. Hereinafter, the vibration component, of which thefrequency is double the frequency of the fundamental wave command Kbase,is referred to as “the second harmonic component”.

The output voltage Vadd and the output power Wadd, in accordance withthe adjustment wave command, are represented by the followingexpressions.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 11} \right\rbrack & \; \\{{Vadd} = {{K\_}{3 \cdot {Vdc} \cdot {\sin\left( {{3 \cdot \omega_{0} \cdot t} + \theta_{N}} \right)}}}} & (11) \\\left\lbrack {{Expression}\mspace{14mu} 12} \right\rbrack & \; \\\begin{matrix}{{Wadd} = {{Vadd} \cdot {Ibase}}} \\{= {{- \frac{{K\_}{3 \cdot {Vdc} \cdot B}}{2}} \cdot}} \\{\left( {{\cos\left( {{4 \cdot \omega_{0} \cdot t} + \theta_{N} - \varphi} \right)} - {\cos\left( {{2 \cdot \omega_{0} \cdot t} + \theta_{N} + \varphi} \right)}} \right)}\end{matrix} & (12)\end{matrix}$

Furthermore, the output power in which the adjustment wave command Kaddis superimposed on the fundamental wave command Kbase, in accordancewith the phase voltage command, is represented by the followingexpression.

$\begin{matrix}{\mspace{79mu}\left\lbrack {{Expression}\mspace{14mu} 13} \right\rbrack} & \; \\{{{Wbase} + {Wadd}} = {{{- \frac{A \cdot B}{2}} \cdot \left( {{\cos\left( {{2 \cdot \omega_{0} \cdot t} - \varphi} \right)} - {\cos(\varphi)}} \right)} - {\frac{{K\_}{3 \cdot {Vdc} \cdot B}}{2} \cdot \left( {{\cos\left( {{4 \cdot \omega_{0} \cdot t} - \varphi + \theta_{N}} \right)} - {\cos\left( {{2 \cdot \omega_{0} \cdot t} + \varphi + \theta_{N}} \right)}} \right)}}} & (13)\end{matrix}$

Assuming that the phase calculation module 122 calculates the initialphase θ_(N) by Expression (5) and the amplitude calculation module 123calculates the adjustment wave amplitude K_3 by Expression (6), thefollowing expression is obtained by substituting Expressions (5) and (6)into Expression (13).

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 14} \right\rbrack & \; \\{{{Wbase} + {Wadd}} = {\frac{A \cdot B}{2} \cdot \left( {{- {\cos\left( {{4 \cdot \omega_{0} \cdot t} - {3 \cdot \varphi}} \right)}} + {\cos(\varphi)}} \right)}} & (14)\end{matrix}$

The output power represented by Expression (14) does not include thesecond harmonic component. In this manner, the second harmonic componentdisappears by calculating the initial phase θ_(N) by Expression (5) andcalculating the adjustment wave amplitude K_3 by Expression (6). Hence,the second harmonic ripple of the DC voltage value Vdc also disappears.

The frequency of the vibration component at the output power representedby Expression (14) is four times the frequency of the fundamental wavecommand Kbase. Hereinafter, the vibration component, of which thefrequency is four times the frequency of the fundamental wave commandKbase, is referred to as “the fourth harmonic component”. As long as thevibration component remains in the output power, the ripple of the DCvoltage value Vdc also remains, but the amplitude of the ripple isreduced as the vibration component becomes the fourth harmonic componentof the high frequency.

In one or more of the above examples, the second harmonic componentdisappears by calculating the initial phase θ_(N) by Expression (5) andcalculating the adjustment wave amplitude K_3 by Expression (6) has beenexemplified. However, in other examples, the second harmonic componentsmay not be cancelled. If the second harmonic component of the outputpower is at least reduced by superimposing the adjustment wave commandKadd on the fundamental wave command Kbase, the second harmonic rippleis suppressed.

The magnetic pole position information acquisition module 131 acquiresinformation indicating the magnetic pole position of the electric motor9. The magnetic pole position acquired by the magnetic pole positioninformation acquisition module 131 corresponds to ω₀·t in Expression(1). The magnetic pole position information acquired by the magneticpole position information acquisition module 131 is used for thegeneration of the fundamental wave command by the fundamental wavegeneration module 112, the coordinate conversion in the coordinateconversion module 115, the calculation of the adjustment wave phase inthe phase calculation module 122, and the like.

(Overmodulation Correction Module)

As described above with reference to the amplitude calculation module123, if the phase voltage command having the amplitude exceeding the DCvoltage value Vdc is output to the inverter circuit such that thedesired three-phase AC power is not obtained, it may be difficult tocontrol the electric motor 9 to a desired state. Therefore, thecontroller 100 may further be configured to include an overmodulationcorrection module 124. The overmodulation correction module 124subtracts the excess amount of amplitude, the amount by which theamplitude of one of the phase voltage commands exceeds the DC voltagevalue Vdc, from the amplitude of all the phase voltage commands. Forexample, if the amplitude of the phase voltage command for the inverter2U exceeds the DC voltage value Vdc, the overmodulation correctionmodule 124 calculates the amount by which the amplitude of the phasevoltage command exceeds the DC voltage value Vdc (hereinafter referredto as “the excess amount”), and subtracts the excess amount from theamplitudes of the phase voltage command for all the inverters 2U, 2V,and 2W.

(Boost Circuit and Boost Control Module)

In order to further increase the voltage amplitude that can be output bythe inverter cell 10, the controller 100 may further include a boostcontrol module 128, and the inverter cell 10 may further include a boostcircuit 20. As the fundamental wave amplitude K rises, the boost controlmodule 128 generates a boost command for raising the DC voltage valueVdc. For example, the boost control module 128 generates the boostcommand in response to the fundamental wave amplitude K exceeding apredetermined reference value (hereinafter referred to as “the secondreference value”). The second reference value may be, for example, themodulation rate threshold value Kth or a value less than the modulationrate threshold value Kth. The second reference value may be less thanthe first reference value.

As described above, in some examples the value of the modulation ratethreshold value Kth may change in accordance with the power factor ofthe three-phase AC power. Therefore, the boost control module 128 maychange the second reference value in accordance with the power factor.For example, the boost control module 128 may decrease the secondreference value as the power factor decreases. Additionally, the boostcontrol module 128 may generate the boost command such that the DCvoltage value Vdc is equal to or greater than the amplitude of the phasevoltage command. For example, if the fundamental wave amplitude Kexceeds the second reference value, the boost control module 128generates the boost command so as to boost the DC voltage value Vdc witha boost rate equal to or greater than the magnification obtained bydividing the fundamental wave amplitude K by the second reference value.

The boost circuit 20 may comprise a circuit for raising the DC voltagevalue Vdc in response to the boost command. For example, the boostcircuit 20 may include a chopper circuit, such as a chopper circuit 21and a chopper driving circuit 22. The chopper circuit 21 includes a coil23, a switch 24, and a diode 25. The coil 23 is provided in the DC bus14P between the rectifier circuit 11 and the capacitor 13. The switch 24is connected to the DC buses 14P and 14N between the coil 23 and thecapacitor 13. The switch 24 switches between a state of storing energyin the coil 23 and a state of releasing stored energy to the capacitor13 side by on/off switching. The diode 25 is provided in the DC bus 14Pbetween the switch 24 and the capacitor 13 and prevents current from thecapacitor 13 to the switch 24.

The chopper driving circuit 22 controls the chopper circuit 21 so as toraise the DC voltage value Vdc. For example, the chopper driving circuit22 controls the chopper circuit 21 so as to periodically switch on/offof the switch 24, and adjusts the boost rate in accordance with theratio of the on time and the off time of the switch 24. For example, thechopper driving circuit 22 calculates the on time and the off time inaccordance with the boost rate per the following expression.

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 15} \right\rbrack & \; \\{{Toff} = \frac{T}{\alpha}} & (15)\end{matrix}$

T: switching cycle

Toff: off time

α: boost rate command value

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 16} \right\rbrack & \; \\{{Ton} = {T\left( {1 - \frac{1}{\alpha}} \right)}} & (16)\end{matrix}$

Ton: on time

For examples comprising the boost circuit 20, the inverter cell 10 mayinclude two voltage detectors 31A and 31B as the voltage detectors 31.The voltage detector 31A detects the DC voltage value Vdc between the DCbuses 14P and 14N. The voltage detector 31B detects the DC voltage valueVdc between the DC bus 14N and the connection point of the choppercircuit 21 and the capacitor 13. In one or more of the followingexamples, the DC voltage value Vdc detected by the voltage detector 31Ais referred to as the DC voltage value Vdc1, and the DC voltage valueVdc detected by the voltage detector 31B is referred to as the DCvoltage value Vdc2, so as to distinguish the DC voltage values Vdc.

(Power Source)

In order to further increase the voltage amplitude that can be output bythe inverter cell 10, the power conversion device 1 may further includea power source 3 (see FIG. 1). For example, the power source 3 generatesan input AC voltage for inputting to the inverter cell 10 with thevoltage amplitude greater than the fundamental wave amplitude K. Forexample, the power source 3 has a transformer 41. The transformer 41 isinterposed between a power system PS and all the inverter cells 10 andgenerates the input AC voltage for each of the inverter cells 10. Forexample, the transformer 41 generates an input AC voltage which isgreater than the maximum value of the fundamental wave amplitude K whichcan be set in the inverter cell 10.

(Hardware Configuration of Controller)

An example hardware configuration of the controller 100 will now bedescribed. As illustrated in FIG. 4, the controller 100 includes acircuit 190. The circuit 190 includes one or more processors 191, amemory 192, a storage 193, and an input/output port 194. The storage 193records a program for configuring each function module of the controller100. The storage 193 may comprise a computer-readable device such as anon-volatile semiconductor memory mounted on a substrate or the like,but may be a hard disk, a magnetic disk, an optical disk, or the likeexternally equipped. The memory 192 temporarily stores the programloaded from the storage 193, the calculation result of the processor191, and the like. The processor 191 may be configured to execute theprogram in cooperation with the memory 192, in order to configure and/orcontrol one or more of the functional modules. The input/output port 194inputs and outputs a signal to and from each inverter cell 10 inresponse to a command from the processor 191.

The circuit 190 may comprise hardware, software, firmware, or anycombination thereof. The circuit 190 may perform one or more of thefunctions through execution of a program. In some examples, the circuit190 may perform at least a part of the functions by the use of one ormore integrated circuits, such as dedicated logic circuits or anapplication specific integrated circuit (ASIC), or the like.

(Power Conversion Procedure)

Subsequently, as an example of the power conversion method, a powerconversion procedure performed by the power conversion device 1 will bedescribed. The power conversion procedure may include generating afundamental wave command for generating one-phase AC power ofthree-phase AC power for each of the inverters 2U, 2V, and 2W so as tocause the inverters 2U, 2V, and 2W to generate three-phase AC power, andgenerating an adjustment wave command having triple the frequency of thefundamental wave command for each of the inverters 2U, 2V and 2W.Additionally, the power conversion procedure may comprise outputting aphase voltage command, in which the adjustment wave command issuperimposed on the fundamental wave command, for each of the inverters2U, 2V, and 2W. The phase of the adjustment wave command may then becalculated based on the power factor of the three-phase AC power so asto reduce the voltage ripple occurring in the DC buses 14P and 14N atdouble the frequency of the fundamental wave command.

FIG. 5 is a block diagram illustrating an example power conversionprocedure. The power conversion device 1 repeats the power conversionprocedure illustrated in FIG. 5 at a preset control cycle. In FIG. 5, atblock B01, the current command generation module 111 generates thecurrent command values Id_ref and Iq_ref. At block B02, the coordinateconversion module 115 converts the current measurement values Iu_fbk,Iv_fbk, and Iw_fbk into the current measurement values Id_fbk andIq_fbk. At the addition point P01, the fundamental wave generationmodule 112 calculates the current deviation by subtracting the currentmeasurement values Id_fbk and Iq_fbk from the current command valuesId_ref and Iq_ref.

At block B03, the fundamental wave generation module 112 performs aproportional calculation, a proportional/integral calculation, aproportional/integral/differential calculation, or the like, on thecurrent deviation, and calculates the voltage command values Vd_ref andVq_ref by adding the counter electromotive force proportional to thefrequency command to the calculation result. For example, thefundamental wave generation module 112 calculates the counterelectromotive force by using the frequency command f_ref. At block B04,the fundamental wave generation module 112 calculates the voltagecommand amplitude A based on the voltage command values Vd_ref andVq_ref. For example, the fundamental wave generation module 112calculates the square root of the square sum of the voltage commandvalues Vd_ref and Vq_ref as the voltage command amplitude A.

At block B05, the fundamental wave generation module 112 calculates thefundamental wave amplitude K by dividing the voltage command amplitude Aby the DC voltage value Vdc2. As described above, the DC voltage valueVdc2 is a value measured at the connection point of the chopper circuit21 and the capacitor 13. Therefore, when the DC voltage value Vdc2 isincreased by the chopper circuit 21, the rise of the fundamental waveamplitude K is prevented.

At block B06, the fundamental wave generation module 112 calculatesfundamental wave commands Kbase_u, Kbase_v, and Kbase_w of the U phase,the V phase, and the W phase by coordinate conversion. At block B07, theamplitude calculation module 123 calculates the adjustment waveamplitude K_3 based on the fundamental wave amplitude K. Specificprocessing contents at block B07 will be described later.

At block B08, the power factor angle calculation module 121 and thephase calculation module 122 calculate the adjustment wave phase φ_3(3·ω₀·t+θ_(N) in Expression (3)) based on the current measurement valuesId_fbk and Iq_fbk and the voltage values Vd and Vq. Additionalprocessing contents at block B08 will be described later.

At block B09, the adjustment wave generation module 113 generates theadjustment wave commands Kadd_u, Kadd_v, and Kadd_w for the U phase, theV phase, and the W phase, respectively, based on the adjustment waveamplitude K_3 calculated at block B07 and the adjustment wave phase φ_3calculated at block B08. At the addition point P02, the command outputmodule 114 adds the adjustment wave commands Kadd_u, Kadd_v, and Kadd_wto the fundamental wave commands Kbase_u, Kbase_v, and Kbase_w,respectively, and outputs the phase voltage command values Kref_u,Kref_v, and Kref_w for the inverters 2U, 2V, and 2W.

At block B11, the overmodulation correction module 124 subtracts theamount, by which one of the phase voltage command values Kref_u, Kref_v,and Kref_w exceeds the DC voltage value Vdc2, from all the phase voltagecommand values Kref_u, Kref_v, and Kref_w, and outputs the correctedcommand values Kout_u, Kout_v, and Kout_w to the cell controllers 40 ofthe inverter cells 10 of the inverters 2U, 2V, and 2W, respectively. Thecell controllers 40 generate the gate driving signals in accordance withthe corrected command values Kout_u, Kout_v, Kout_w, and output thegenerated gate driving signals to the gate driving circuits 15.Therefore, the switching circuit 16 is controlled so as to generate theoutput voltage in accordance with the corrected command values Kout_u,Kout_v, and Kout_w. Additional processing contents at block B11 will bedescribed later.

At block B12, the boost control module 128 calculates the fundamentalwave amplitude K by dividing the voltage command amplitude A generatedat block B04 by the DC voltage value Vdc1. At block B13, the boostcontrol module 128 generates the boost rate command value α based on thefundamental wave amplitude K. Additional processing contents at blockB13 will be described later. As described above, the DC voltage valueVdc1 is a value measured at the connection point of the rectifiercircuit 11 and the chopper circuit 21. Therefore, at block B13, theboost rate command value α is generated based on the DC voltage valueVdc before boosting by the chopper circuit 21.

Additional example contents of the amplitude calculation procedure atblock B07, the phase calculation procedure at block B08, theovermodulation correction procedure at block B11, and the boost commandgeneration procedure at block B13 are described below.

(Amplitude Calculation Procedure)

FIG. 6 is a block diagram illustrating an example amplitude calculationprocedure. In FIG. 6, at block B21, the first amplitude calculationmodule 125 calculates the first amplitude having the same value as thefundamental wave amplitude K. At block B22, the second amplitudecalculation module 126 multiplies the fundamental wave amplitude K bythe slope “a” of the approximate descending line L21. At the additionpoint P11, the second amplitude calculation module 126 calculates thesecond amplitude by adding the intercept “b” of the approximatedescending line L21 to the value generated at block B22. At the additionpoint P12, the amplitude selection module 127 calculates thedetermination reference value “u” by subtracting the first referencevalue Kth1 from the fundamental wave amplitude K.

At block B23, the amplitude selection module 127 selects one of thefirst amplitude and the second amplitude as the adjustment waveamplitude K_3 in accordance with the value of the determinationreference value “u”. For example, if the determination reference value“u” is equal to or less than zero, the amplitude selection module 127selects the first amplitude as the adjustment wave amplitude K_3. If thedetermination reference value “u” exceeds zero, the second amplitude isselected as the adjustment wave amplitude K_3. The amplitude calculationprocedure is completed.

(Phase Calculation Procedure)

FIG. 7 is a block diagram illustrating an example phase calculationprocedure. In FIG. 7, at block B31, the power factor angle calculationmodule 121 calculates the phase angle of the voltage values Vd and Vq.At block B32, the power factor angle calculation module 121 calculatesthe phase angle of the current measurement values Id_fbk and Iq_fbk. Atthe addition point P21, the power factor angle calculation module 121calculates the power factor angle by subtracting the phase angle of thecurrent measurement values Id_fbk and Iq_fbk from the phase angle of thevoltage values Vd and Vq.

At block B33, the phase calculation module 122 performs the filteringprocessing of first order delay on the power factor angle calculated atthe addition point P21. A time constant Tc at block B33 is set to, forexample, a value greater than a carrier cycle of the gate drivingcircuit 15. At block B34, the phase calculation module 122 calculatesthe initial phase θ_(N) by multiplying the power factor angle by 2. Atthe addition point P22, the phase calculation module 122 converts thephase angle of the voltage values Vd and Vq in the dq coordinate systeminto the phase angle in the fixed coordinate system (for example, the αβcoordinate system).

At block B35, the phase calculation module 122 calculates the phaseangle corresponding to triple the phase angle of the fundamental wavecommand Kbase (hereinafter referred to as “the triple phase angle”) bymultiplying the phase angle obtained at the addition point P22 by 3. Atthe addition point P23, the phase calculation module 122 calculates theadjustment wave phase φ_3 by subtracting the initial phase θ_(N)obtained at block B34 from the triple phase angle obtained at block B35.The phase calculation procedure is then completed.

(Overmodulation Correction Procedure)

FIG. 8 is a block diagram illustrating an example overmodulationcorrection procedure. In FIG. 8, at block B41, the overmodulationcorrection module 124 selects the maximum value Kmax of the phasevoltage command values Kref_u, Kref_v, and Kref_w. More specifically,the overmodulation correction module 124 selects the maximum value ofthe phase voltage command value of the U phase, the phase voltagecommand value of the V phase, and the phase voltage command value of theW phase. At block B42, the overmodulation correction module 124 performsthe limiter processing to set the upper limit value to 1 with respect tothe maximum value Kmax.

At the addition point P31, the overmodulation correction module 124subtracts the maximum value Kmax from the result of the limiterprocessing at block B42. If the maximum value Kmax is 1 or less, theresult of the limiter processing at block B42 has the same value as themaximum value Kmax. If the maximum value Kmax exceeds 1, the result ofthe limiter processing at block B42 becomes 1, which is less than themaximum value Kmax. Therefore, if the maximum value Kmax is 1 or less,the calculation result at the addition point P31 becomes zero, and ifthe maximum value Kmax exceeds 1, the calculation result at the additionpoint P31 becomes a negative value. Hereinafter, the calculation resultat the addition point P31 is referred to as “the first correctionvalue”.

At block B43, the overmodulation correction module 124 selects theminimum value Kmin of the phase voltage command values Kref_u, Kref_v,and Kref_w. In some examples, the overmodulation correction module 124selects the minimum value of the phase voltage command value of the Uphase, the phase voltage command value of the V phase, and the phasevoltage command value of the W phase. At block B44, the overmodulationcorrection module 124 performs the limiter processing to set the lowerlimit value to −1 with respect to the minimum value Kmin.

At the addition point P32, the overmodulation correction module 124subtracts the minimum value Kmin from the result of the limiterprocessing at block B44. If the minimum value Kmin is −1 or more, theresult of the limiter processing at block B44 has the same value as theminimum value Kmin. If the minimum value Kmin is less than −1, theresult of the limiter processing at block B44 becomes −1, which isgreater than the minimum value Kmin. Therefore, if the minimum valueKmin is equal to or greater than −1, the calculation result at theaddition point P32 becomes zero, and if the minimum value Kmin is lessthan −1, the calculation result at the addition point P32 becomes apositive value. Hereinafter, the calculation result at the additionpoint P32 is referred to as “the second correction value”.

At the addition point P33, the overmodulation correction module 124calculates the determination reference value “u” by adding thecalculation results at the addition points P31 and P32. At block B45,the overmodulation correction module 124 selects one of the firstcorrection value and the second correction value in accordance with thedetermination reference value “u”. For example, the overmodulationcorrection module 124 selects the first correction value when thedetermination reference value “u” is equal to or less than zero, andselects the second correction value when the determination referencevalue “u” exceeds zero.

At the addition point P34, the overmodulation correction module 124 addsthe correction value selected at block B45 to the phase voltage commandvalues Kref_u, Kref_v, and Kref_w. For example, if the maximum valueKmax exceeds 1 and the minimum value Kmin is −1 or more, the firstcorrection value becomes a negative value and the second correctionvalue becomes zero. Therefore, the determination reference value “u”becomes a negative value, the first correction value is selected, andthe first correction value which is a negative value is added to all thephase voltage command values Kref_u, Kref_v, and Kref_w. Therefore, theamount by which the maximum value Kmax exceeds 1 is subtracted from thephase voltage command values Kref_u, Kref_v, and Kref_w. Therefore, Kmaxcan be set to 1 or less. On the other hand, if the maximum value Kmax is1 or less and the minimum value Kmin is less than −1, the firstcorrection value becomes zero and the second correction value becomes apositive value. Therefore, the determination reference value “u” becomesa positive value, the second correction value is selected, and thesecond correction value which is a positive value is added to all thephase voltage command values Kref_u, Kref_v, and Kref_w. Therefore, theamount (lower amount) by which the minimum value Kmin exceeds −1 issubtracted from the phase voltage command values Kref_u, Kref_v, andKref_w. Therefore, Kmin can be set to −1 or more.

(Boost Command Generation Procedure)

FIG. 9 is a block diagram illustrating an example boost commandgeneration procedure. In FIG. 9, at block B51, the boost control module128 sets the boost rate command value to 1. Hereinafter, the calculationresult in block B51 is referred to as “the first command value”. Atblock B52, the boost control module 128 generates the boost rate commandvalue by dividing the fundamental wave amplitude K by the secondreference value Kth2. Hereinafter, the generation result at block B52 isreferred to as “the second command value”. At the addition point P41,the boost control module 128 calculates the determination referencevalue “u” by subtracting the second reference value Kth2 from thefundamental wave amplitude K.

At block B53, the boost control module 128 selects either one of thefirst command value or the second command value as the boost ratecommand value α in accordance with the determination reference value u.For example, if the determination reference value u is equal to or lessthan zero, the boost control module 128 selects the first command valueas the boost rate command value α. When the determination referencevalue u exceeds zero, the second command value is selected as the boostrate command value α. Therefore, if the fundamental wave amplitude Kexceeds the second reference value Kth2, the boost rate command value αis calculated so as to boost the DC voltage value Vdc with themagnification obtained by dividing the fundamental wave amplitude K bythe second reference value Kth2. The boost command generation procedureis then completed.

In one or more examples, and as described above, the power conversiondevice 1 may include the three inverters 2U, 2V, and 2W which convertthe DC power of the DC buses 14P and 14N into the single-phase AC power,and the controller 100 which controls the three inverters 2U, 2V, and 2Wso as to generate the three-phase AC power. The controller 100 mayinclude the fundamental wave generation module 112 which generates thefundamental wave command for each of the inverter 2U, 2V, and 2W, forgenerating the AC power of one phase of the three-phase AC power, andthe adjustment wave generation module 113 which generates the adjustmentwave commands with triple the frequency of the fundamental wave commandfor each of the inverters 2U, 2V, and 2W. Additionally, the controller100 may include the command output module 114 which outputs the phasevoltage command, in which the adjustment wave command is superimposed onthe fundamental wave command, for each of the inverters 2U, 2V, and 2W,and the phase calculation module 122 which calculates the phase of theadjustment wave command based on the power factor of the three-phase ACpower so as to reduce the voltage ripples occurring in the DC buses 14Pand 14N at double the frequency of the fundamental wave command.

When the adjustment wave command having triple the frequency of thefundamental wave command is superimposed on the fundamental wavecommand, the single-phase AC voltage waveforms output from therespective inverters 2U, 2V, and 2W change. On the other hand, thewaveform of the line voltage of the three-phase AC power is the same asin the example in which the adjustment wave command is not superimposed.In some examples, the adjustment wave component superimposed on onephase voltage by the adjustment wave command and the adjustment wavecomponent superimposed on the other phase voltage by the adjustment wavecommand cancel each other. Therefore, by superimposing the adjustmentwave command, the voltage waveform of each phase may be adjusted withoutsubstantially changing the three-phase AC power.

In some examples, the voltage ripple tends to occur at double thefrequency of the fundamental wave command on the DC bus of the inverter.Hereinafter, this is referred to as “the second harmonic ripple”. Inorder to suppress the second harmonic ripple, a large capacitor capacitymay be maintained between the DC buses 14P and 14N. Therefore, in orderto reduce the capacitor capacity, the second harmonic ripple may bereduced by measures other than the capacitor capacity. For example, thesecond harmonic ripple can be reduced by adjusting the phase of theadjustment wave command with respect to the fundamental wave command(hereinafter referred to as “the adjustment wave phase”). The adjustmentwave phase which is effective for reducing the second harmonic ripplecorrelates with the power factor of the three-phase AC power. Usingthese properties, the phase calculation module 122 calculates the phaseof the adjustment wave command with respect to the fundamental wavecommand based on the power factor of the three-phase AC power so as toreduce the second harmonic ripple. Therefore, since the second harmonicripple is reduced, the capacitor capacity can be reduced. Therefore, thecapacitor capacity of the inverters 2U, 2V and 2W may be reduced.

The controller 100 may further include the amplitude calculation module123 which calculates the amplitude of the adjustment wave command basedon the amplitude of the fundamental wave command so as to reduce thevoltage ripple. The amplitude of the adjustment wave command(hereinafter referred to as “the adjustment wave amplitude”) which iseffective for reducing the second harmonic ripple correlates with theamplitude of the fundamental wave command. Therefore, according to theconfiguration of calculating the amplitude of the adjustment wavecommand based on the amplitude of the fundamental wave command so as toreduce the voltage ripple, the second harmonic ripple may be morereliably reduced.

The controller 100 may further include the overmodulation correctionmodule 124 which subtracts the excess amount of amplitude, the amount bywhich the amplitude of one of the phase voltage commands exceeds thevoltage of the DC buses 14P and 14N, from the amplitude of all the phasevoltage commands. Adjusting the phase and the amplitude of theadjustment wave command, so as to reduce the second harmonic ripple, maycause the phase voltage command to exceed the voltage of the DC bus.Hereinafter, if the phase voltage command exceeds the voltage of the DCbus, it is referred to as “the overmodulation state”. In theovermodulation state, the deviation between the actually output phasevoltage and the phase voltage command increases, and the reliability ofthe three-phase AC power decreases. On the other hand, since theovermodulation correction module 124 is further included, the occurrenceof the overmodulation state can be prevented and the reliability of thethree-phase AC power can be improved.

In order to reduce the voltage ripple, the amplitude calculation module123 may include the first amplitude calculation module 125 whichcalculates the amplitude of the adjustment wave command based on theamplitude of the fundamental wave command, and the second amplitudecalculation module 126 which lowers the amplitude of the adjustment wavecommand in response to the increase in the amplitude of the fundamentalwave command. Additionally, the amplitude calculation module 123 mayinclude the amplitude selection module 127 which switches the amplitudeof the adjustment wave command from the amplitude calculated by thefirst amplitude calculation module 125 to the amplitude calculated bythe second amplitude calculation module 126 in response to the increasein the amplitude of the fundamental wave command.

The occurrence of the overmodulation state can be prevented by switchingthe amplitude of the adjustment wave command from the amplitudecalculated by the first amplitude calculation module 125 to theamplitude calculated by the second amplitude calculation module 126. Inthis manner, the method of preventing the occurrence of theovermodulation state by switching the amplitude is referred to as “thefirst overmodulation prevention”. On the other hand, the method ofpreventing the occurrence of the overmodulation state by theovermodulation correction module 124 (the method of subtracting theamount by which the phase voltage command exceeds the voltage of the DCbuses 14P and 14N, from the phase voltage command) is referred to as“the second overmodulation prevention”.

According to the second overmodulation prevention, since the amount bywhich the phase voltage command exceeds the voltage of the DC bus(hereinafter referred to as “the overvoltage amount”) is subtracted fromthe phase voltage command, the overmodulation can be prevented with highreliability. On the other hand, since the voltage exceeding amount isforcibly subtracted from the phase voltage command, the waveform of theadjustment wave command which provides the reduction effect of thesecond harmonic ripple is disturbed. This may lessen or impair theeffect of reducing the second harmonic ripple. On the other hand,according to the first overmodulation prevention, since the waveform ofthe adjustment wave command is maintained, the effect of reducing thesecond harmonic ripple may not be lessened due to the disturbance of thewaveform. Therefore, the overmodulation prevention can be achieved withhigh reliability while maintaining the effect of reducing the secondharmonic ripple by combining the first and second overmodulationpreventions.

The controller 100 further includes the boost control module 128 whichgenerates the boost command for raising the voltage of the DC bus inresponse to the increase in the amplitude of the fundamental wavecommand, and each of the three inverters 2U, 2V, and 2W may include theboost circuit which raises the voltage between the DC buses 14P and 14Nin response to the boost command. In this case, the allowable range ofthe amplitude of the fundamental wave command can be widened, and thesecond harmonic ripple can be more reliably reduced.

The amplitude selection module 127 switches the amplitude of theadjustment wave command from the amplitude calculated by the firstamplitude calculation module 125 to the amplitude calculated by thesecond amplitude calculation module 126 in response to the amplitude ofthe fundamental wave command which exceeds the first reference value.Additionally, the boost control module 128 may generate the boostcommand in response to the amplitude of the fundamental wave commandwhich exceeds the second reference value less than the first referencevalue. In this case, since the voltage between the DC buses 14P and 14Nrises prior to the first overmodulation prevention, the second harmonicripple may be more reliably reduced. In addition, when the voltage riseof the DC buses 14P and 14N is insufficient, the overmodulationprevention can be achieved with high reliability while preventing thereduction in the effect of reducing the second harmonic ripple bycombining the first and second overmodulation preventions.

The boost control module 128 may generate the boost command such thatthe voltage between the DC buses 14P and 14N are equal to or greaterthan the amplitude of the phase voltage command. In this case, thesecond harmonic ripple may be more reliably reduced.

The power conversion device 1 may further include the power source 3which generates the input AC voltage to be input to the inverters 2U,2V, and 2W at the voltage amplitude greater than the amplitude of thefundamental wave command. In this case, the allowable range of theamplitude of the fundamental wave command can be widened, and the secondharmonic ripple can be more reliably reduced.

Each of the three inverters 2U, 2V, and 2W may be a series connectedmulti-level inverter in which the AC sides of a plurality of invertercircuits 12 are connected in series.

It is to be understood that not all aspects, advantages and featuresdescribed herein may necessarily be achieved by, or included in, any oneparticular example embodiment. Indeed, having described and illustratedvarious examples herein, it should be apparent that other examples maybe modified in arrangement and detail.

We claim all modifications and variations coming within the spirit andscope of the subject matter claimed herein.

Regarding the above embodiments, the following appendices are appended.

[Appendix 1] A power conversion device comprising:

three inverters configured to convert DC power of a DC bus intosingle-phase AC power; and

a controller configured to control the three inverters so as to generatethree-phase AC power,

wherein the controller comprises:

a fundamental wave generation module configured to generate afundamental wave command for generating one-phase AC power constitutingthe three-phase AC power for each of the inverters;

an adjustment wave generation module configured to generate anadjustment wave command having triple the frequency of the fundamentalwave command for each of the inverters;

a command output module configured to output a phase voltage command, inwhich the adjustment wave command is superimposed on the fundamentalwave command, for each of the inverters; and

a phase calculation module configured to calculate a phase of theadjustment wave command based on a power factor of the three-phase ACpower so as to reduce a voltage ripple occurring in the DC bus at doublethe frequency of the fundamental wave command.

[Appendix 2] The power conversion device according to appendix 1,wherein the controller further comprises an amplitude calculation moduleconfigured to calculate an amplitude of the adjustment wave commandbased on an amplitude of the fundamental wave command so as to reducethe voltage ripple.

[Appendix 3] The power conversion device according to appendix 2,wherein the controller further comprises an overmodulation correctionmodule configured to subtract an amount, by which an amplitude of one ofthe phase voltage commands exceeds a voltage of the DC bus, fromamplitudes of all the phase voltage commands.

[Appendix 4] The power conversion device according to appendix 2 or 3,wherein the amplitude calculation module comprises:

a first amplitude calculation module configured to calculate theamplitude of the adjustment wave command based on the amplitude of thefundamental wave command so as to reduce the voltage ripple;

a second amplitude calculation module configured to reduce the amplitudeof the adjustment wave command in response to the increase in theamplitude of the fundamental wave command; and

an amplitude selection module configured to switch the amplitude of theadjustment wave command from an amplitude calculated by the firstamplitude calculation module to an amplitude calculated by the secondamplitude calculation module in response to the increase in theamplitude of the fundamental wave command.

[Appendix 5] The power conversion device according to appendix 4,wherein the controller further comprises a boost control moduleconfigured to generate a boost command to raise the voltage of the DCbus in response to the increase in the amplitude of the fundamental wavecommand, and

each of the three inverters comprises a boost circuit configured toraise the voltage of the DC bus in response to the boost command.

[Appendix 6] The power conversion device according to appendix 5,wherein the amplitude selection module is configured to switch theamplitude of the adjustment wave command from the amplitude calculatedby the first amplitude calculation module to the amplitude calculated bythe second amplitude calculation module as the amplitude of thefundamental wave command exceeds a first reference value, and

the boost control module is configured to generate the boost command asthe amplitude of the fundamental wave command exceeds a second referencevalue less than the first reference value.

[Appendix 7] The power conversion device according to appendix 2 or 3,wherein the controller further comprises a boost control moduleconfigured to generate a boost command to raise the voltage of the DCbus in response to the increase in the amplitude of the fundamental wavecommand, and

each of the three inverters comprises a boost circuit configured toraise the voltage of the DC bus in response to the boost command.

[Appendix 8] The power conversion device according to any one ofappendices 5 to 7, wherein the boost control module is configured togenerate the boost command such that the voltage of the DC bus is equalto or greater than the amplitude of the phase voltage command.

[Appendix 9] The power conversion device according to any one ofappendices 2 to 8, further comprising a power source configured togenerate an input AC voltage to be input to the inverters at a voltageamplitude greater than the amplitude of the fundamental wave command.

[Appendix 10] The power conversion device according to any one ofappendices 1 to 9, wherein each of the three inverters is a seriesconnected multi-level inverter in which AC sides of a plurality ofinverter circuits are connected in series.

[Appendix 11] A power conversion method comprising:

generating a fundamental wave command for generating one-phase AC powerof three-phase AC power for each of three inverters, such that thethree-phase AC power is generated in the three inverters configured toconvert DC power of a DC bus into single-phase AC power;

generating, for each of the three inverters, an adjustment wave commandhaving triple the frequency of the fundamental wave command;

outputting, for each of the three inverters, a phase voltage command, inwhich the adjustment wave command is superimposed on the fundamentalwave command; and

calculating a phase of the adjustment wave command with respect to thefundamental wave command based on a power factor of the three-phase ACpower so as to reduce a voltage ripple occurring in the DC bus at doublethe frequency of the fundamental wave command.

FIG. 10 is a flow chart illustrating an example power conversion methodfor generating one-phase AC power of three-phase AC power for each ofthree inverters configured to convert DC power of a DC bus intosingle-phase AC power. The operations of the method may be performed foreach of the three inverters. Additionally, the operations may beperformed by a controller in response to execution of instructionsstored on a non-transitory memory device.

At operation 210, a fundamental wave command is generated. Thefundamental wave command has a fundamental frequency.

At operation 215, an initial phase of the adjustment wave command isdetermined to be offset from an initial phase of the fundamental wavecommand so as to reduce a voltage ripple occurring in the DC bus atdouble the fundamental frequency of the fundamental wave command.

At operation 220, an adjustment wave command is generated, having triplethe fundamental frequency of the fundamental wave command.

At operation 225, a phase voltage command is output, in which theadjustment wave command is superimposed on the fundamental wave command.

FIG. 11 is a flow chart illustrating a flow chart illustrating anotherexample power conversion method for generating AC power.

Operations 210 to 225 is the same as FIG. 10. At operation 235, a boostcommand is generated to raise the voltage of the DC bus in response toan increase in the amplitude of the fundamental wave.

At operation 240, the voltage of the DC bus is raised in response to theboost command.

At operation 245, the amplitude voltage value corresponding to the firstreference value is raised in response to the boost command.

At operation 250, a modification operation of the amplitude of theadjustment wave is switched between the first amplitude modificationoperation and the second amplitude modification operation in response tothe amplitude of the fundamental wave command exceeding a firstreference value.

At operation 255, an amplitude of the adjustment wave command isincreased in a first amplitude modification operation in response to anincrease in the amplitude of the fundamental wave.

At operation 260, the amplitude of the adjustment wave command isreduced in a second amplitude modification operation in response to anincrease in the amplitude of the fundamental wave.

What is claimed is:
 1. A power conversion device comprising: threeinverters each of which is configured to convert DC power of a DC businto single-phase AC power; and a controller configured to control thethree inverters so as to generate three-phase AC power, wherein thecontroller is further configured to, for each of the three inverters:generate a fundamental wave for generating one-phase AC powerconstituting the three-phase AC power, the fundamental wave having afundamental frequency; generate an adjustment wave having a frequencythat is triple the fundamental frequency of the fundamental wave;increase an amplitude of the adjustment wave in a first amplitudemodification operation in response to an increase in an amplitude of thefundamental wave in a first range; reduce the amplitude of theadjustment wave in a second amplitude modification operation in responseto an increase in the amplitude of the fundamental wave in a secondrange; switch between the first amplitude modification operation and thesecond amplitude modification operation in response to the amplitude ofthe fundamental wave exceeding a predetermined threshold between thefirst range and the second range; add the adjustment wave to thefundamental wave to generate a phase voltage, wherein an initial phaseof the adjustment wave prior to generating the phase voltage is offsetfrom an initial phase of the fundamental wave so as to reduce a voltageripple occurring in the DC bus at a frequency that is double thefundamental frequency of the fundamental wave; and output the phasevoltage to generate the three-phase AC power.
 2. The power conversiondevice according to claim 1, wherein a phase angle of the initial phaseof the adjustment wave is less than four times a power factor angle ofthe three-phase AC power.
 3. The power conversion device according toclaim 1, wherein a phase angle of the initial phase of the adjustmentwave is between 1.8 times and 2.2 times a power factor angle of thethree-phase AC power.
 4. The power conversion device according to claim1, wherein the controller is configured to modify the amplitude of theadjustment wave to be between 0.5 times and 1.5 times the amplitude ofthe fundamental wave.
 5. The power conversion device according to claim1, wherein the controller is configured to gradually reduce theamplitude of the adjustment wave in the second amplitude modificationoperation such that the amplitude of each phase voltage is keptapproximately equal to a voltage of the DC bus.
 6. The power conversiondevice according to claim 1, wherein the controller is furtherconfigured to: generate a boost command to raise the voltage of the DCbus in response to the increase in the amplitude of the fundamentalwave, and for each of the three inverters, raise the voltage of the DCbus in response to the boost command.
 7. The power conversion deviceaccording to claim 1, further comprising a power source configured togenerate an input AC voltage to be input to the inverters at a voltageamplitude greater than the amplitude of the fundamental wave, whereineach of the three inverters comprises a rectifier circuit rectifying theinput AC voltage and outputting the rectified voltage to the DC bus. 8.The power conversion device according to claim 1, wherein each of thethree inverters comprises a series connected multi-level inverter inwhich AC sides of a plurality of inverter circuits are connected inseries.
 9. The power conversion device according to claim 6, wherein thecontroller is configured to switch between the first amplitudemodification operation and the second amplitude modification operationin response to the amplitude of the fundamental wave exceeding a firstreference value, and wherein the boost command is generated in responseto the amplitude of the fundamental wave exceeding a second referencevalue less than the first reference value.
 10. The power conversiondevice according to claim 6, wherein the controller is configured togenerate the boost command such that the voltage of the DC bus is equalto or greater than the amplitude of the phase voltage.
 11. The powerconversion device according to claim 9, wherein the controller isconfigured to raise an amplitude voltage value of the fundamental wavecorresponding to the first reference value in response to the boostcommand.
 12. A power conversion device comprising: three inverters eachof which is configured to convert DC power of a DC bus into single-phaseAC power; and a controller configured to control the three inverters soas to generate three-phase AC power, wherein the controller is furtherconfigured to, for each of the three inverters: generate a fundamentalwave for generating one-phase AC power constituting the three-phase ACpower, the fundamental wave having a fundamental frequency; generate anadjustment wave having a frequency that is triple the fundamentalfrequency of the fundamental wave; add the adjustment wave to thefundamental wave to generate a phase voltage, wherein an initial phaseof the adjustment wave prior to generating the phase voltage is offsetfrom an initial phase of the fundamental wave so as to reduce a voltageripple occurring in the DC bus at a frequency that is double thefundamental frequency of the fundamental wave; determine a difference involtage between a maximum voltage associated with an amplitude of thephase voltage and a voltage of the DC bus; subtract the difference involtage from the amplitude of all the phase voltages; and output thephase voltage to generate the three-phase AC power.
 13. A powerconversion method comprising: generating a fundamental wave forgenerating one-phase AC power of three-phase AC power for each of threeinverters configured to convert DC power of a DC bus into single-phaseAC power, the fundamental wave having a fundamental frequency;generating, for each of the three inverters, an adjustment wave having afrequency that is triple the fundamental frequency of the fundamentalwave; increasing an amplitude of the adjustment wave in a firstamplitude modification operation in response to an increase in theamplitude of the fundamental wave; reducing the amplitude of theadjustment wave in a second amplitude modification operation in responseto an increase in the amplitude of the fundamental wave; switchingbetween the first amplitude modification operation and the secondamplitude modification operation in response to the amplitude of thefundamental wave exceeding a predetermined threshold; adding, for eachof the three inverters, the adjustment wave to the fundamental wave togenerate a phase voltage, wherein an initial phase of the adjustmentwave is offset from an initial phase of the fundamental wave so as toreduce a voltage ripple occurring in the DC bus at double thefundamental frequency of the fundamental wave; and outputting, for eachof the three inverters, the phase voltage to generate the three-phase ACpower.
 14. The method according to claim 13, wherein the amplitude ofthe adjustment wave is modified based on the amplitude of thefundamental wave so as to further reduce the voltage ripple.
 15. Themethod according to claim 13, further comprising: generating a boostcommand to raise the voltage of the DC bus in response to the increasein the amplitude of the fundamental wave, and for each of the threeinverters, raising the voltage of the DC bus in response to the boostcommand.
 16. A non-transitory memory device having instructions storedthereon that, in response to execution by a controller, cause thecontroller to perform operations comprising: generating a fundamentalwave for generating one-phase AC power of three-phase AC power for eachof three inverters, each of the three inverters configured to convert DCpower of a DC bus into single-phase AC power, the fundamental wavehaving a fundamental frequency; generating, for each of the threeinverters, an adjustment wave having a frequency that is triple thefundamental frequency of the fundamental wave; increasing an amplitudeof the adjustment wave in a first amplitude modification operation inresponse to an increase in the amplitude of the fundamental wave;reducing the amplitude of the adjustment wave in a second amplitudemodification operation in response to an increase in the amplitude ofthe fundamental wave; switching between the first amplitude modificationoperation and the second amplitude modification operation in response tothe amplitude of the fundamental wave exceeding a predeterminedthreshold; adding, for each of the three inverters, the adjustment waveto the fundamental wave to generate a phase voltage, wherein an initialphase of the adjustment wave with respect to the fundamental wave isoffset from an initial phase of the fundamental wave so as to reduce avoltage ripple occurring in the DC bus at double the fundamentalfrequency of the fundamental wave; and outputting, for each of the threeinverters, the phase voltage to generate the three-phase AC power.